Re: Not quite managing my first join

On Apr 9, 2005 3:49 PM, Rodolfo J. Paiz <rpaiz(at)simpaticus(dot)com> wrote:
>
> Hi!
>
> I'm within a hair of getting my first join to work, but something is
> wrong and for the life of me I can't figure out why. Hints, anyone?
>
> ====================================================================
> select t1.month,t1.single,t2.multi from (
> select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as single
> from flights,aircraft
> where flights.callsign=aircraft.callsign and aircraft.engines=1
> group by month order by month) as t1
> full outer join (
> select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as multi
> from flights,aircraft
> where flights.callsign=aircraft.callsign and aircraft.engines=2
> group by month order by month) as t2
> on (t1.month = t2.month);
>
> month | single | multi
> ---------+--------+-------
> 2003-08 | 5.6 |
> 2003-09 | 3.5 |
> 2003-10 | 4.2 |
> 2003-11 | 17.7 |
> 2003-12 | 16.4 |
> 2004-01 | 13.3 |
> 2004-02 | 1.7 | 7.6
> 2004-03 | 12.2 |
> 2004-04 | 13.3 | 4.1
> 2004-05 | 7.3 |
> 2004-06 | 7.1 | 9.4
> 2004-07 | 2.8 | 7.6
> 2004-08 | 6.0 | 7.0
> 2004-09 | 7.9 | 2.4
> 2004-10 | 2.5 |
> | | 17.4
> 2004-12 | 5.8 | 3.1
> | | 10.1
> | | 14.1
> 2005-03 | 9.5 | 2.7
> (20 rows)
> ====================================================================
>
> The problem is that the query seems to be returning all the correct
> results, *BUT* I have two problems:
>
> - Instead of zero where I did not fly a multi-engine airplane, it
> shows no value at all.
>
> - For columns where t1 had no value, no month is shown.
>
> How can I fix those two little details? My brain is temporarily fried
> and I'm not understanding the docs very clearly.
>
> Thanks!
>
> --
> Rodolfo J. Paiz <rpaiz(at)simpaticus(dot)com>
>
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>
Well it looks like your outer join is doing exactly like it should.
>From the docs at
http://www.postgresql.org/docs/8.0/interactive/queries-table-expressions.html
"FULL OUTER JOIN

First, an inner join is performed. Then, for each row in T1 that does not
satisfy the join condition with any row in T2, a joined row is added with
null values in columns of T2. Also, for each row of T2 that does not satisfy
the join condition with any row in T1, a joined row with null values in the
columns of T1 is added."
Here is a simple example that shows your problem and how you can turn those
nulls into something

CREATE TABLE child
(
child_id int8,
child_name varchar(50)
) ;

CREATE TABLE parent
(
parent_id int8,
parent_name varchar(50),
child_id int8
) ;

insert into parent values (1,'Mommy Kim',89);--HAS A CHILD
insert into parent values (1,'Mommy Lisa',NULL);--HAS NO CHILD
insert into child values (89,'Child Chucky');--HAS A MOTHER
insert into child values (109,'Child Bruce');--HAS NO MOTHER

SELECT p.parent_name,c.child_name
FROM public.parent as p
FULL JOIN public.child as c ON p.child_id = c.child_id;

SELECT CASE WHEN p.parent_name IS NULL
THEN 'HAS NO CHILD' ELSE p.parent_name
END,
CASE WHEN c.child_name IS NULL
THEN 'HAS NO PARENT' ELSE c.child_name
END
FROM public.parent as p
FULL JOIN public.child as c ON p.child_id = c.child_id;